1. Field of the Invention
The field of the present invention is fiber moldings for use in the production of fiber-reinforced composite materials such as fiber-reinforced metallic, plastic and ceramic members, and particularly processes for forming fiber moldings with short fibers oriented in one direction, such as silicon carbide whiskers, short carbon fibers and short alumina fibers.
2. Description of the Prior Art
There is such conventionally known processes for forming fiber moldings, wherein a slurry molding material containing short fibers dispersed in a liquid is molded using a compression molding process, a vacuum molding process or the like to produce a fiber molding, as disclosed in Japanese Patent Application Laid-open No. 166934/86.
In the fiber moldings produced by the above molding processes, the short fibers are oriented perpendicular to the pressing direction, but within the plane perpendicular the pressing direction, the short fibers are oriented randomly.
If the composite material is desired to be reinforced in an axial direction, then it is necessary to form a fiber molding with the short fibers oriented in the axial direction. This requirement, however, cannot be satisfied in the above prior art processes.
In addition, if the composite material includes an annular or semi-annular portion or the like provided in a portion thereof, then it is necessary to shape an annular portion or the like even in the fiber molding. In this situation, it is desirable that the short fiber in the annular portion or the like of the fiber molding be oriented along an arc of the annular portion or the like and that the volume fraction (Vf) of the short fiber be uniformly maintained over the entire annular portion or the like, for the purpose of improving the mechanical properties of the composite material. Such properties, however, cannot be obtained by the above prior art process.
When the strength of the composite material is intended to be improved in a particular portion the volume fraction of the short fiber in that portion of the fiber molding which corresponds to such portion of the composite material must be increased. Thereupon, there is employed means for secondarily pressing such portion of the fiber molding produced in the above process from the same direction as in the above molding operation.
In the fiber molding produced in this manner, however, the short fiber in such portion of the fiber molding is oriented at random within the plane perpendicular to the pressing direction and thus intertwined. For this reason, in some case, the molten matrix cannot sufficiently penetrate into such portions of the fiber molding during production of a composite material, and consequently, it is feared that defects may be produced in the resulting composite material.
A procedure used to form fiber moldings for fiber-reinforced composite materials having a hollow portion is to pour a slurry molding material consisting of short fibers dispersed in a liquid into a mold cavity and then form the fiber molding while removing the liquid through a porous core to shape the hollow portion.
Such porous cores have pores substantially uniformly dispersed over the whole thereof, and hence, the liquid permeability of the entire porous core is generally even.
Consequently, when the molding material in the cavity is pressed during molding, the running of the molding material is developed preferentially in a section of the porous core closer to its upper end, resulting in a higher volume fraction of the short fiber in that section closer to the upper end, and in a lower volume fraction of the short fiber in a section closer to a base end of the porous core.
To obtain a fiber-reinforced metallic member reinforced with short fibers, wherein the flexibility of the shape and the volume fraction of the short fibers are increased and the short fibers are oriented in one direction, it is desirable to employ a single-step pressure casting process rather than a pressure casting process using a fiber molding. Such a technique, however, is still not developed at present.